Various packaging techniques have been used to build a load of unit products and subsequently wrap them for transportation, storage, containment and stabilization, protection and waterproofing. One system uses stretch wrapping machines to stretch, dispense and wrap stretch packaging material around a load. Stretch wrapping can be performed as an inline, automated packaging technique which dispenses and wraps packaging material in a stretch condition around a load on a pallet to cover and contain the load. Pallet stretch wrapping, whether accomplished by a turntable, rotating arm, vertical rotating ring, or horizontal rotating ring, typically covers the four vertical sides of the load with a stretchable film such as polyethylene film. In each of these arrangements, relative rotation is provided between the load and the packaging material dispenser to wrap packaging material about the sides of the load.
Stretch wrapping machines provide relative rotation between a stretch wrap packaging dispenser and a load either by driving the stretch wrap packaging dispenser around a stationary load or rotating the load on a turntable. Upon relative rotation, packaging material is wrapped on the load. Ring style stretch wrappers generally include a roll of packaging material mounted in a dispenser which rotates about the load on a ring. Wrapping rings are categorized as vertical rings or horizontal rings. Vertical rings move vertically between an upper and lower position to wrap film around a load. In a vertical ring, as in turntable and rotating wrap arm apparatuses, the four vertical sides of the load are wrapped, along the height of the load. Horizontal rings are stationary and the load moves through the ring, usually on a conveyor, as the dispenser rotates around the load to wrap packaging material around the load. In the horizontal ring, the length of the load is wrapped. As the load moves through the ring and off of the conveyor, the packaging material slides off of the conveyor (surface supporting the load) and into contact with the load.
Historically, ring style wrappers have suffered from excessive film breaks and limitations on the amount of containment force applied to the load (as determined in part by the amount of prestretch used) due to erratic speed changes required to wrap “non-square” loads, such as narrow, tall loads, short, wide loads, and short, narrow loads. The non-square shape of such loads often results in the supply of excess packaging material during the wrapping cycle, during time periods in which the demand rate for packaging material by the load is exceeded by the supply rate of the packaging material by the dispenser. This leads to loosely wrapped loads. In addition, when the demand rate for packaging material by the load is greater than the supply rate of the packaging material by the dispenser, breakage of the packaging material may occur.
When stretch wrapping a typical rectangular load, the demand for packaging material varies, decreasing as the packaging material approaches contact with a corner of the load and increasing after contact with the corner of the load. When wrapping a tall, narrow load or a short load, the variation in the demand rate is even greater than in a typical rectangular load. In vertical rings, high speed rotating arms, and turntable apparatuses, the variation is caused by a difference between the length and the width of the load. In a horizontal ring apparatus, the variation is caused by a difference between the height of the load (distance above the conveyor) and the width of the load.
The amount of force, or pull, that the packaging material exhibits on the load determines how tightly and securely the load is wrapped. Conventionally, this force is controlled by controlling the feed or supply rate of the packaging material dispensed by the packaging material dispenser with respect to the demand rate of packaging material required by the load. Efforts have been made to supply the packaging material at a constant tension or at a supply rate that increases as the demand rate increases and decreases as the demand rate decreases. However, when variations in the demand rate are large, fluctuations between the feed and demand rates result in loose packaging of the load or breakage of the packaging material during wrapping.
Prior art solutions utilize a change in the force on the packaging material to signal the need for a change in the supply rate. In response to an increase in the force acting on the film, the speed of the film payoff will be increased. In response to a decrease in the force acting on the film, the speed of the film payoff will be decreased. Reliance on sensing a change in the force on the packaging material means that a response to the need for a change in the supply rate is not initiated until after the change in demand rate has occurred. These prior art devices react to the change in the demand rate, they cannot anticipate or act simultaneously with the change. Thus, there is a lag between the time the demand rate changes and the time the supply rate changes to meet the changed demand rate. The elasticity of the packaging material can exacerbate this problem.
Due to a design preference to eliminate electrical connections to the moving ring of ring style wrappers, force sensing/reacting solutions used in other types of wrapping apparatus are not feasible. In addition, a high rate of change in film demand when wrapping non-square loads, for example, short loads, requires an immediate change in payoff speed of the packaging material in order to prevent either excess payoff or breakage. Existing force feedback systems cannot effectively react within the time frame necessary to prevent excess distribution of film or breakage.
Various spring-loaded film accumulators (also known as dancer accumulators) have been designed in an effort to resolve this problem. Such accumulators vary the supply rate to generally correspond with that of the demand rate by “taking up” excess or slack packaging material supplied during low demand periods. Such devices have met with only limited success.
Other devices, such as friction brakes and powered prestretch devices have been used to attempt to prevent excess packaging material distribution and breakage. Problems with these existing devices are discussed in the background of U.S. Pat. Nos. 4,676,048 and 4,953,336, which are incorporated herein by reference.
Friction brake systems provide a roll of film within a film carriage supported on a core shaft. The film is dispensed due to the relative rotation between the load and the packaging material dispenser, i.e., as rotation occurs, the film is pulled off of the roll to be wrapped around the load. Thus, film dispensing is not driven, but is passive. The film is stretched by the application of a brake directly to core of the film roll as the film is dispensed. Such friction brake systems were popular due to their simplicity. However, such systems had several drawbacks. One such drawback was the change in wrap force as the roll of film changed size. That is, as the film was dispensed, the size of the film roll necessarily decreased, and at the same time, the force being applied to the load by the stretch wrap (wrap force) increased. In addition, the roll of film was part of a stretching “tug of war,” and thus all of the imperfections of the film winding process (nicks, burrs, “feathering”) would cause the film to break prematurely. Also, the friction brake could not accommodate acceleration and deceleration in payoff demand due to the load corners.
Film driven roller stretch devices were created to address the problems associated with friction brake systems. In film driven roller stretch devices, such as those disclosed in U.S. Pat. Nos. 4,302,920 and 4,497,159, both assigned to Lantech, Inc., and incorporated herein by reference, the packaging material is stretched between two interconnected rollers, one moving faster than the other. These rollers may be connected by friction, as disclosed in U.S. Pat. No. 4,497,159, where the two prestretch rollers are in frictional contact with one another via a cam assembly. This eliminated many of the problems associated with friction brake devices. However, in order to accommodate irregular and force sensitive loads, it was necessary to find a way to vary the wrap force of the film as it was applied to the load. This problem was addressed by the device disclosed in U.S. Pat. No. 4,302,920, where the rollers were connected via a gear/clutch assembly to allow variation in the wrap force.
In this device, the substantial changes in demand speed are transmitted directly from the load back through the packaging material to the pre-stretch device, so that the supply speed of the film moving across the downstream roller to the load changes accordingly. However, the entire force exerted between the rollers is applied to the rollers by the packaging material being wrapped around the load, and prestretch device inertia causes a phase delay or lag in supply need changes. The elasticity of the packaging material between the downstream roller and the load adds to the lag. In addition, any hole or imperfection in the packaging material causes a weakening between the load and the prestretch mechanism, and potentially resulting in breakage of the packaging material.